EAS270_Ch3_BehaviourAtmos_C.odp Sec 3.6 Influence of temperature on pressure distribution JDW, EAS Ualberta, last mod. 21 Sept. 2016 Thermal pressure system: If one column Now (realistically) shallow area of high or low warmed relative to allowing the pressure created by cooling or the other, but no horizontal pressure warming lateral motion gradient to result in permitted.... motion.... Recall (Eq 3.10): Δ P P g 1 = − Δ z Rd T v

Fig 3.16 Sec 3.6 Influence of temperature on pressure distribution 2/11

● thermal pressure systems are shallow – pressure gradient weakens quickly with increasing height, and reverses aloft

● "core" of a thermal low (high) is warm (cold)

● example of a "meso-scale" thermal Fig 11.25a circulation: sea breeze (faster daytime warming of land surface than of ocean

depth of a meso- Fig 3.17 scale thermal circulation nominally ~ 1 km

As aside: Terminology in relation to scale of circulation system 3/11

● microscale circulation – horizontal These are qualitative designations – scale up to ~ 1km boundary between categories subjective

● mesoscale ("middle scale") circulation – horizontal scales from On the synoptic scale, the temperature ~1km to ~100 km contrast between the oceanic surface and wintertime continental land surface may ● synoptic scale circulation – induce what is effectively a "thermal high" horizontal scale from ~100km to whose horizontal scale is vast. ~1000km These features are so common that they ● planetary scale show up when we draw isobars of climatological January surface pressure Isobars of climatological January sea level press. & surface wind

equiv. to Fig 12.3a Fig 12.3a Sec 3.6 Contrasting "dynamic" versus "thermal" pressure systems 4/11

● when meteorologists speak of there being "no weather," they mean that the situation is quiescent – no precip, nothing radical going on, perhaps no change at all (other than that due to the rising and falling of the sun – the diurnal cycle). "No weather" does not necessarily mean it is warm, or not cold

● "dynamic" pressure systems, particularly affecting middle latitudes, are associated with "weather", that is, they are responsible for changeable weather... that (in the case of dynamic lows, i.e. "storms") may entail clouds & precipitation, and perhaps even violently windy conditions

● dynamical lows entail a pattern of disturbance spanning the whole troposphere – unlike thermal lows, dynamic lows are "deep"

● dynamic lows (at least sometimes) can be considered to have a "cold core" – whereas thermal lows are warm-cored

Tues 20 Sept. 2016, 1236 MDT Sec 3.6 Contrasting shallow "thermal" and deep "dynamic" pressure systems 5/11

What do the blue lines represent?

Fig 3.18a Dynamic (cold core) low Fig 3.18b Dynamic (warm core) high

Fig 3.173.17a Thermal Thermal (warm (warm core) core) low low Fig 3.17b Thermal (cold core) high

Sec 3.7 Column temperature (largely) controls height of isobaric surfaces 6/11

Think of the 500 hPa isobaric surface as an imaginary sheet in the sky – at every point on the sheet, P=500 hPa. The sheet may be horizontal, but in general it is not – nor, unlike in this image, is it necessarily evenly sloped. It is a complex "topographic" surface featuring ridges and valleys, just like the ground surface

Fig 3.22a Sec 3.7 Highs and lows on the isobaric charts (here 500 hPa level) 7/11

Stippled region:

● Dashed lines the 1000- thickness lies between 534 and 540 dam 500 hPa thickness ∆z ● considered that in such areas precip, IF occurring, could be in form of freezing Stippling 534-540 hPa rain

● NOT a statement that freezing rain is probable Δ z[dam]=2T v [K]

Fig 3.22b Sec 3.7 250 hPa isobaric chart – polar front jetstream 8/11

height ~ 10 km

isotachs

jet cores

depth of system in Gulf

Fig 3.23 Sec 3.7 Dynamic storms are deep: CMC surface analysis for 12Z May 7, 2012 9/11 Sec 3.7 Dynamic storms are deep: CMC analyses for 12Z May 7, 2012 10/11

700 hPa 250 hPa, Fig 3.23

850 hPa 500 hPa, Fig 3.22 Sources of data aloft 11/11

● radiosondes provide T, Td and (if tracked by radar) wind speed and direction

● satellites, geostationary and polar orbiting – ysm provide (i) images and (ii) "soundings" (that zxs yyq return information wse indirectly related to yqd temperature and wlw humidity throughout the atmosphere)

● weather radar – precip. and (if dopplerized) storm winds

● instrument packages on commercial aircraft Lecture of 21 Sept.

● thermal vs. dynamic pressure systems A "zero-hour prog" (P0H) is effectively the same as an analysis. ● terminology for the "scale" of "circulation systems" RDPS=Regional Deterministic ● more familiarization Prediction System with upper charts – what they give us

● significance of the stippled 534-540 hPa thickness band on CMC charts

● jetstream

This low over Hudson Bay has wrapped cooler air (easily identified by the gray 534-540 dam thickness This is a common chart, showing: band) into N. Alberta & Primary field – "MSLP" Secondary – thickness Saskatchewan MSLP – surface pressure corrected to sea-level